Process for preparing a polyamide, a nylon salt to be used therein and a process for making the salt

ABSTRACT

Processes for preparing a polyamide, a polyamide material obtained by such processes, a nylon salt to be used therein, and processes for making the nylon salt are disclosed. The processes for preparing the polyamide include a direct solid state polymerization process. The salt used herein is a granulate material, prepared by a process, comprising spraying an aqueous solution comprising a diamine and a dicarboxylic acid dissolved in an aqueous medium, or an aqueous slurry comprising a diammonium dicarboxylate salt dispersed in an aqueous medium directly obtained from such an aqueous solution, onto or in a fluidized bed of diammonium dicarboxylate salt particles, while evaporating the aqueous medium, thereby forming a granulate salt material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of commonly owned U.S. Ser. No.16/778,822, filed Jan. 31, 2020 (now U.S. Pat. No. 11,299,589), which isa divisional of U.S. Ser. No. 15/329,270, filed Jan. 26, 2017 (now U.S.Pat. No. 10,584,206), which is the U.S. national phase of InternationalApplication No. PCT/EP2015/073639 filed Jul. 29, 2015 which designatedthe U.S. and claims priority to EP Patent Application No. 14178927.1filed Jul. 29, 2014, the entire contents of each of which are herebyincorporated by reference.

FIELD

The present invention relates to a process for preparing a polyamide, apolyamide material obtainable by the said process, a nylon salt to beused therein, and a process for making the nylon salt.

BACKGROUND AND SUMMARY

A nylon salt is herein understood to be a diammonium dicarboxylate salt,also indicated as a diamine/dicarboxylic acid salt, in other words asalt prepared from a diamine or a mixture of diamines, and adicarboxylic acid or a mixture of dicarboxylic acids. Polyamidesprepared from such salts are generally indicated as AA-BB polyamides.Herein an AA-BB polyamide is understood to be a polyamide comprisingalternating AA and BB repeating units, wherein AA represents therepeating units derived from diamine and BB represents the repeatingunits derived from dicarboxylic acid. Throughout this text the wordings“diammonium dicarboxylate salt” and “nylon salt” are meant to have thesame meaning and are therefore interchangeable.

The process for preparing a polyamide of concern in the presentinvention is a direct solid state polymerization process. Such a processmakes use of solid nylon salts, and throughout such process the salt andresulting polyamide and any intermediate prepolymer product thereofremain in the solid state. For the wording “direct solid statepolymerization”, also the abbreviation “DSSP” will be used herein.

In such a process, the salt used is typically in a powder or particularshape, and the aim is also to obtain the resulting polymer in a powderor particle shape. The salt used herein can be a salt powder orgranulate, obtained for example, by spray drying, precipitation fromsolution, or a dry route process involving reaction of liquid diaminewith solid dicarboxylic acid. The salt may also have a particular shapeof compacted powder particles. Such a process is described, for example,in the patents U.S. Pat. No. 5,128,442 and GB-801,733.

In a DSSP process, the salt powder or granulate is heated to a reactiontemperature below the melting temperature of the salt, thereby inducingpolymerization of the salt and formation of the polyamide. While keepingthe polymerization temperature below the melting point of the salt andthe resulting polymer and its intermediates, the polymer is generallyobtained, or at least intended to be obtained as a granulate material.Such a DSSP process is suitably carried out in a static bed reactor, ormoving packed bed reactor. However, when carried out on large scale thiscan easily lead to agglomeration or caking of the powder or particulatematerial, thereby complicating the discharging of the polymer productfrom the reactor used. Another problem is linked to the heat transferneeded for heating the salt to the reaction temperature and for coolingthe polymer before discharging. Because of the solid state andparticulate form of the salt heat transfer is less efficient than inother processes wherein, for example, liquid melts or solutions areused. Limited heat transfer during polymerization can lead to anundesirable temperature profile throughout the reactor content,resulting in material varying in temperature history and consequently ina product with an inhomogeneous material composition. Heat transferallowing for homogeneous heating is less a problem on small scale, i.e.on laboratory scale, but becomes more difficult at larger scales.Heating efficiency can be increased by using a process with a stirredreaction mass. Stirring may be accomplished in a reactor equipped with amechanical agitator, such as a stirrer, or in tumbling reactor. Aproblem with direct solid state polymerization of solid nylon saltsinvolving agitation and mixing of particulate material is that such aprocess, when applied on larger scale, still suffers from occurrence ofagglomeration of particles, resulting in formation of much biggerparticles or even lumps, or caking and scaling of the reactor, orsuffers from extensive attrition of particles, resulting in fines, oreven suffers from a combination of these problems. Fines create problemsin further processing such as dusting, which creates an explosion risk,and limited flowability. Elimination of problems with dust in downstreamprocessing would require additional measures for removing fines anddust. Lump formation can create problems in further processing, such asneed for classification and grinding. Scaling on the wall can lead toreduced heat transfer and can thus complicate heating or cooling of thereactor content. This may lead to partially non-reacted material atspots at which heat transfer has been less efficient. Scaling onmechanical parts, such as on stirrers, can make stirring less efficient.With lump formation and scaling on valves, also discharging from thereactor can become more difficult. Scaling can also result in fouling ofthe reactor. It can result in residues in the reactor, which uponprolonged exposure to reaction conditions, such as elevated temperature,can lead to inhomogeneity in product properties, and to impurities, oreven dark particles in the final product. To prevent build-up ofscaling, leading to excessive scaling and further intensification of theabove described associated problems, the reactor wall would have to becleaned frequently, which will cause severe downtime of the plant.

An aim of the present invention is therefore to provide a process forpreparing a polyamide wherein the above problems of agglomeration, finesand scaling are reduced at least in part or eventually fully absent.Another aim is to provide a nylon salt, that can be used in the saidprocess, wherein the above problems are reduced at least in part oreventually fully absent. A further aim is to provide a nylon salt thatreduces or eventually fully eliminates above problems, when used in adirect solid state polymerization process. A further aim is to provide apolyamide material that can be produced in an efficient manner, is easyto handle and requires less extensive and time consuming steps forcleaning and de-dusting.

These aims have been achieved with the following aspects of the presentinvention.

The first aspect concerns a process for preparing a nylon salt, whichaccording to the present invention is prepared by fluidized bed spraygranulation.

The second aspect of the invention concerns a nylon salt, wherein thesalt is obtained by preparation in a fluidized bed spray granulationprocess.

The third aspect concerns a process for preparing a polyamide comprisingdirect solid state polymerization of a nylon salt, wherein the salt is agranulate salt material prepared by fluidized bed spray granulation.

A further aspect concerns a polyamide granulate material, comprising anAA-BB polyamide obtained by direct solid state polymerization of adiammonium dicarboxylate salt, the salt used being a salt prepared byfluidized bed spray granulation process.

The effect of the first and second aspect of the invention, being afluidized bed spray granulation process and nylon salt obtained by saidprocess, is that the salt is a granulate salt material that can be usedin a direct solid state polymerization process while showing lessproblems in a direct solid state polymerization (DSSP) process comparedto nylon salts in the form of granular or granulate material withparticles of similar size and the same composition prepared via otherroutes. Discharging of the polymer produced by DSSP from the reactor hasshown to be easier. Fouling of the reactor is reduced, while at the sametime, agglomeration and formation of fines is limited. Reducedagglomeration and formation of fines is also demonstrated withanalytical data on particle size distribution, showing that the particlesize distributions of starting salt material and final polymer powderdiffer less when compared with those of polyamide granulate materialobtained by DSSP from nylon salts of the same composition prepared viaother routes. Furthermore, the polyamide material prepared by DSSP fromthe salt obtained by fluidized bed spray granulation, is produced in anefficient manner since it requires less extensive steps for cleaning andde-dusting, and is easy to handle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) and 1(b) are photographs of a salt according to the inventionwherein the salt particles tend to have a blackberry like structure,whereby FIG. 1(a) is a microscopic photograph of the salt particles witha relative small average particle size and FIG. 1(b) is a photograph ofthe salt particles under further enlargement;

FIG. 2 is a microscopic photograph of salt particles according to theinvention with a relative large average particle size, visibly largerthan those of FIGS. 1(a) and (b) whereby the particles have a more orless rounded off structure;

FIG. 3 is a microscopic photograph of a salt prepared by spray dryingaccording to a comparative experiment whereby the salt particles have akind of puffed open popcorn type of structure or little droplets thatare burst open; and

FIG. 4 is a microscopic photograph of a salt prepared by dosing liquiddiamine to a solid dicarboxylic acid according to a comparativeexperiment whereby the salt particles have a potato shape with a crackedsurface like dried mud with many sub-micron crystallites.

DETAILED DESCRIPTION

The process according to the invention for preparing the nylon salt,i.e. the diammonium dicarboxylate salt, is a fluidized bed spraygranulation process. The process comprises spraying of an aqueoussolution comprising a diamine, or a mixture of diamines, and adicarboxylic acid or a mixture of dicarboxylic acids dissolved in anaqueous medium, or spraying an aqueous slurry comprising a diammoniumdicarboxylate salt dispersed in an aqueous medium directly obtained froman aqueous solution comprising a diamine and a dicarboxylic aciddissolved in an aqueous medium, onto or in a fluidized bed of diammoniumdicarboxylate salt particles, wherein a preheated gaseous medium is ledthrough the fluidized bed, while evaporating the aqueous medium, therebyforming a granulate salt material.

It is noted that the diamine used in the process for preparing the nylonsalt can be a single diamine, or a mixture of different diamines. Unlessspecifically noted otherwise, the single form diamine has the meaning ofincluding both a single diamine and a mixture of different diamines.Analogously, the dicarboxylic acid used in the process for preparing thenylon salt can be a single dicarboxylic acid, or a mixture of differentdicarboxylic acids. Unless specifically noted otherwise, the single formdicarboxylic acid has the meaning of including both a singledicarboxylic acid and a mixture of different dicarboxylic acid.

The expression “in the range of X-Y”, wherein X and Y represent lowerand upper limits, has the meaning that the range is from X up to andincluding Y. In other words, in said expression the values of bothlimits are included in the range.

With a granular or granulate material is herein understood a materialconsisting for at least 50 wt. % of particles with a particle size of atmost 3 mm (millimeter). Such a material has a d50 of 3 mm or less.Herein the amount of particles with a particle size of at most 3 mm isdetermined by sieving and weighing. The particle size for the d50 value,respectively the wt. % of particles with a particle size of at most 3 mmis determined by the method according to DIN 66-165 part 1 and 2.

The slurry that can be used in the inventive salt preparation process issuitably directly obtained from an aqueous solution comprising a diamineand a dicarboxylic acid. This means that first the diamine and thedicarboxylic acid have to be completely dissolved in the aqueous medium,for example at elevated temperature, and then the slurry is formed, forexample by cooling the solution. By cooling, first the saturation levelfor the dissolved salt will be reached and upon further cooling goingbeyond the saturation level, crystallization of the salt will occur. Theadvantage of preparing the slurry in this manner is that in the processfor preparing the polyamide by DSSP, the amount of residual unreacteddicarboxylic acid, if any, is lower.

The aqueous medium comprised by the slurry or solution used in theinventive process comprises at least water. It may suitably comprise aco-solvent, such as an organic solvent. Preferably the co-solvent is aliquid that is miscible with water. Suitably the liquid miscible withwater is an alcohol. Examples thereof include methanol and ethanol. Suchliquids miscible with water may be used for example to increase thesolubility of the salt in water. This can be advantageous for examplewith less hydrophilic components, such as longer chain diamines. Theliquid miscible with water may also be used otherwise, for example topartially precipitate the salt from an aqueous solution, thereby forminga slurry. Suitably, the co-solvent is present in an amount of 0-50 wt.%, and more particular 0-25 wt. %, relative to the total weight of waterand co-solvent. In a preferred embodiment, the aqueous medium consistsof water. This has the advantage that no special precautions have to betaken for removal, separation or recycling of any co-solvent.

The aqueous solution or aqueous slurry used in the process according tothe invention can have a salt concentration varying over a wide range,for example in the range from 2.5-80 wt. %. The concentration may wellbe in the range of 1-2.5 wt. %, or even below 1 wt. %. However, such lowconcentration would make the process less efficient in terms of energyconsumption involved in the evaporation of the water. Suitably, thesolution or slurry has a salt concentration of at least 5 wt. %, inparticular at least 10 wt. %. Preferably, the salt concentration is inthe range of 20-80 wt. %, still more preferably 25-70 wt. %. Herein thesalt concentration is calculated as the weight percentage (wt. %) oftotal amount of diamine and dicarboxylic acid, relative to the totalweight of the solution or slurry. For clarity: where in the process amixture of diamines and/or a mixture of dicarboxylic acid is, for thecalculation of the total amount of diamine and dicarboxylic acid, allthe diamines in the mixture and all the dicarboxylic acids in themixture have to be included. The maximum concentration will in practicebe limited by the solubility of the salt. A higher minimum saltconcentration has the advantage that less water has to be evaporated

The aqueous solution or aqueous slurry may have a temperature varyingover a wide range. When solubility allows, the temperature of theaqueous solution or aqueous slurry may well be about room temperature,i.e. about 20° C. or even lower. The temperature may be as high as theboiling point of water at atmospheric pressure, more particular 100° C.,or even above 100° C., when the solution is pressurized. When apressurized solution is used, spraying onto the fluidized powder bedresults in partial evaporation of water upon release of the pressure,while removal of the remaining of the water will take place in the bedupon drying of droplets precipitated on other particles. For thepressurized conditions, suitably a temperature of at most 150° C. and awater content of at least 25 wt. %, relative to the total weight of thesolution are applied.

Preferably said solution or slurry temperature is at least 35° C. and atmost 100° C., and more preferably in the range of 40-95° C. An advantageof a higher solution or slurry temperature is that a higher saltconcentration can be applied, making the drying in the process moreefficient, and allowing for use of gas with a lower temperature. Use ofa temperature below 100° C. avoids the need for a pressure reactor and apressurized solution.

The solution or slurry may comprise, next to the nylon salt,respectively the constituting diamine and dicarboxylic acid, one or moreother components. In principle any component can be present, as long assuch component does not prohibit the formation of solid salt particlesin the fluidized bed spray granulation process and the polymerization ofthe nylon salt in a DSSP process. Suitably, such other components, ifpresent at all, are selected from components that are useful during thepolymerization, or are useful in the polyamide composition to beprepared. Preferably, the solution or slurry comprises at least onecomponent selected from polymerization catalysts, stabilizers and inertfillers. If used at all, the amount of other components is preferablylimited.

The aqueous solution or slurry suitably consists of

-   -   a) 20-90 wt. % of the aqueous medium;    -   b) 10-80 wt. % of salt, either dissolved or dispersed in the        aqueous medium; and    -   c) 0-25 wt. % of other components;        -   wherein the weight percentages (wt. %) are relative to the            total weight of the aqueous solution or slurry, and wherein            the sum of a), b) and c) amounts to 100 wt. %.

Preferably, the aqueous solution or slurry consists of

-   -   a) 20-90 wt. % of the aqueous medium;    -   b) 10-80 wt. % of salt, either dissolved or dispersed in the        aqueous medium;    -   c) 0-2.5 wt. % of at least one polymerization catalyst and/or at        least one stabilizers; and    -   d) 0-22.5 wt. % of other components;        -   wherein the weight percentages (wt. %) are relative to the            total weight of the aqueous solution or slurry, and wherein            the sum of a), b), c) and d) amounts to 100 wt. %.

The preparation of granulate salt material by fluidized bed spraygranulation is supposed to proceed as follows: a salt solution or saltslurry is sprayed onto a fluidized bed of particles. Droplets formed bythe spraying can form a liquid bridge between two or more particles,thereby forming small agglomerates. Other droplets formed by thespraying may dry to form new small particles. These new small particlesmay be bridged together to form new small agglomerates or may beincorporated into other agglomerates, thereby growing theseagglomerates, and ultimately resulting in larger agglomerates.Meanwhile, the solvent in the liquid bridges will evaporate therebysolidifying the smaller and larger agglomerates. The larger agglomeratesmay reach a certain size at which the particles become more difficult tofluidize. Such larger particles might be allowed to settle or be removedfrom the fluidized bed. The remaining smaller agglomerates in thefluidized bed will grow to larger agglomerates, while in the meantimenew small agglomerates will have formed and thus allow the process to becontinued. The larger agglomerates thus formed typically have a kind ofblackberry structure. When growing even larger, the agglomerates maybecome more rounded off, but can still show remaining images of thesmaller particles bound together in the larger agglomerates.Additionally a different way of growing larger particles may occur,namely by deposition of multiple droplets of the solution or slurry onthe surface of a particle, which deposited droplets spread and dry,thereby leading to a kind of onion like structure, at least so at thesurface layers. Thus, the granulate salt material obtained by theprocess according to the invention suitably comprises particles havingan agglomerated shape or an onion like shape.

Suitably, the process is started with preparing a fluidized bed ofprecursor particles. For the precursor particles, in principle anyparticulate material that can be fluidized can be used. For instance, ifthe process is carried out as a semi-continuous process, the salt withthe precursor particles produced in the initial stage may be removed. Incase of a batch wise production, where removal is not that easy, theprecursor particles are suitably selected from materials that may becomprised by the polyamide composition ultimately being produced.

Preferably, the precursor particles consist of nylon salt. The advantageis that such precursor salt can be copolymerized with the nylon saltobtained from the salt solution. More preferably the precursor saltparticles have the same composition as the nylon salt obtained from thesalt solution.

Suitably, the precursor particles are salt particles obtained by spraydrying. Once granulate salt material obtained from the inventive processof fluidized bed spray granulation is available, also such granulatesalt material may be used as precursor particles for preparing thefluidized bed at the start or restart of the process.

A fluidized bed can be created starting from a precursor material inparticle form, and using a gaseous medium for fluidization of theprecursor particles. The gaseous medium is also used for drying of thesolution or slurry sprayed in or onto the fluidized bed.

The gaseous medium led through the fluidized bed is suitably preheated.This contributes to a more effective drying of the sprayed solution orslurry. It preferably has a temperature of at least 50° C., moreparticular at least 70° C. This has the advantage that the waterevaporates faster and drying is more efficient. Herein the temperatureof the gaseous medium is measured before entering into the fluidizedbed. More preferably, the temperature is in the range of 80-250° C., inparticular in the range of 100-200° C. Too high of a temperature for thegaseous medium may lead to degradation of one of its constituents and/orcoloration of the salt product or, and eventually may turn the productyellow. A lower maximum temperature has the advantage that a salt withless to none coloration is obtained.

For aliphatic salts the maximum gas temperature is preferably evenlower, e.g. below 175° C., more particular below 150° C. This reducesthe risk of sticking of the salt particles formed on hot parts in theequipment. Aliphatic salts generally have a lower melting point thantheir semi-aromatic counterparts.

Aliphatic salt are herein understood diammonium dicarboxylate saltsconsisting of aliphatic diamine and aliphatic dicarboxylic acid.

Semi-aromatic salt are herein understood diammonium dicarboxylate saltscomprising at least an aromatic constituent and at least an aliphaticconstituent. Suitably, the semi-aromatic salt consists of aromaticdiamine and aliphatic dicarboxylic acid, or consisting of aliphaticdiamine and aromatic dicarboxylic acid, or consisting of diamine anddicarboxylic acid, wherein at least one of the diamine and thedicarboxylic acid is partially aromatic and partially aliphatic. Thediamine is herein considered to be partially aromatic and partiallyaliphatic, when it consists of at least two diamines, comprising atleast one aromatic diamine and at least one aliphatic diamine. Thedicarboxylic acid is herein considered to be partially aromatic andpartially aliphatic, when it consists of at least two dicarboxylicacids, comprising at least one aromatic dicarboxylic acid and at leastone aliphatic dicarboxylic acid.

The diamine can also be fully aliphatic and comprising at least twodifferent aliphatic diamines. The dicarboxylic acid can also be fullyaromatic and comprising at least two different aromatic dicarboxylicacids.

The salt may also be fully aromatic, i.e. consisting of an aromaticdicarboxylic acid and an aromatic diamine. Example of suitable aromaticsalts are MXDT and PXDT. Such a salt can be dissolved or prepared fromits constituent diamines meta-xylenediamine (MXD) respectivelypara-xylenediamine (PXD) and terephthalic acid (T) as carboxylic acid,in water or in an aqueous liquid, can be isolated from an aqueoussolution by fluidized bed spray granulation, and can also be polymerizedby direct solid state polymerization.

After the gaseous medium has passed through the fluidized bed, it can beremoved as off-gas. The off-gas can be exhausted or recycled, or acombination thereof such that a part is exhausted and another part isrecycled. When being recycled, it may also be used to be fed back intothe spray granulation process. Suitably, off-gas resulting from thegaseous medium led through the fluidized bed, is passed through a filteror through a scrubber, or is dried, or any combination thereof, beforebeing exhausted or recycled.

For the gaseous medium, any gas suitable for fluidization and drying maybe used. Preferably, the gaseous medium is an inert gas, preferablynitrogen. Air or oxygen in combination with very fine particlesconstitutes an explosion risk. Use of inert gas has the advantage of areduced explosion risk.

The temperature of the fluidized bed can be varied over a wide range, aslong as the temperature remains below the melting temperature of thenylon salt. The fluidized bed suitably has a temperature of at least 35°C., whereas for low melting salts an even lower temperature may beapplied. Suitably, the bed temperature is also at least 50° C. below themelting point of the salt.

The terms melting point and melting temperature are herein understood tohave the same meaning, and therefore interchangeable. With the meltingpoint, or melting temperature of the salt is herein understood thetemperature, measured by the DSC method according to ISO-11357-1/3,2011, on pre-dried samples in an N2 atmosphere with heating and coolingrate of 10° C./min. Herein Tm has been calculated from the peak value ofthe highest melting peak in the first heating cycle. Herein thegranulate salt material is dried during 24 hours at 105° C. and under avacuum of 100 mmHg.

Herein the bed temperature is measured in the center of the bed.Preferably, said bed temperature is in the range of 40-100° C., inparticular in the range of 50-80° C. A higher bed temperature has theadvantage that the salt in the bed will dry faster and better and theparticulate salt material produced will have a lower residual moisturecontent. A lower maximum temperature is favorably applied for lowermelting salts, such as for aliphatic salts or semi-aromatic saltscomprising aliphatic dicarboxylic acid.

Suitably, the fluidized bed has a temperature of at least 35° C., whilethe aqueous solution or aqueous slurry has a temperature of at least 35°C. and the gaseous medium led through the fluidized bed has atemperature of at least 50° C.

The temperature of the fluidized bed can be steered, for example byselecting a concentration, a temperature and a spraying speed for thesolution or slurry and adjusting the temperature and throughput of thegaseous medium.

Suitably, the granulate salt material has a residual moisture content ofat most 7.5 wt. % or better at most 5 wt. %, preferably at most 4 wt. %,more preferably at most 2 wt. %, still more preferably at most 1 wt. %.Herein the weight percentage (wt. %) is relative to the total weight ofthe granulate salt material including the residual moisture. Theadvantage of a lower water content is reduced risk of caking uponstorage. With moisture is herein understood water, and where applicableany co-solvent, retained in the granulate salt material obtained afterthe fluidized bed spray granulation process. The residual moisturecontent is measured by determining the weight loss of the granulate saltmaterial upon drying of the salt during 24 hours at 105° C. and under avacuum of 100 mmHg. The residual moisture content is calculated from theweight loss expressed as weight percentage relative to the weight of thegranulate salt material before drying.

The residual water content of the granulate salt material as obtainedfrom the fluidized bed spray granulation can be further reducedafterwards. The residual water content may be reduced, for example, bydrying the granulate salt material in a fluidized bed. For the drying ina fluidized bed, conventional fluid bed units and fluid bed technologyfor drying of particles—powders, agglomerates or pellets—can be used.Herein, for example, process air is supplied through a specialperforated distributor plate and flows through the bed of solids at avelocity sufficient to support the weight of particles in a fluidizedstate.

The particle size of the granulate salt material is the result of abalance between several process parameters. Increasing the residencetime of the particles in the granulator (e.g. by increasing the volumeof the fluidized bed and/or decreasing the outflow of the granulate)will generally result in coarser particles. Suitably, in the fluidizedbed spray granulation process, a classification leg or classificationchannel with classification air is used. Herein, the classification aircan be adjusted in such a way, that particles with the required particlesize can leave the reactor and smaller particles stay in the fluidizedbed. Therefore, a higher gas flow rate in the classification leg willlead to larger particles.

Suitably, the granulate salt material has a particle size distributionwith a median particle size (d50) in the range of 100-3000 μm, moreparticular in the range of 150-2000 μm. Preferably the d50 is in therange of 200-1000 μm, more preferably 250-750 μm. Herein the particlesize distribution is measured with laser granulometry by the methodaccording to ISO 13320-1 at 20° C. The advantage of having a particlesize in the preferred range is that it combines good flowability withgood polymerization characteristics.

The process according to the invention may be carried out inconventional apparatus for fluidized bed spray granulation. The processmay be carried out, for example, in a vertical set-up comprising asubstantially round fluidized bed apparatus with a classificationchannel, or in an essentially horizontal set-up comprising a flowchannel, and applying a horizontally moving bed as a plug flow. Duringor after the fluidized bed spray granulation, the material located inthe fluidized bed or discharged therefrom may be subjected to aconventional classification process. In the horizontal set-up theparticle size is controlled by adjusting the dosing rate versus thespeed of the plug flow.

Fines separated in a classification step may be reused, for example, asprecursor particles for preparing a fluidized bed at the start of theprocess. The start of the process may be start of a new batch in avertical set-up, or the initial part of a horizontally moving bed in anessentially horizontal set-up.

Preferably, the process is carried out in a vertical set-up comprising asubstantially round fluidized bed apparatus with a classificationchannel. The advantage thereof is that control of the particle size isdone in one go by adjusting the speed of the gas flow in theclassification channel in the vertical set-up, meanwhile resulting in anarrow particle size distribution without the need for a separate stepfor the separation of fines.

In the process according to the invention, in principle any nylon saltcan be prepared provided that the salt, or the constituent componentsthereof, can be dissolved in an aqueous medium and an aqueous solutionof the constituting diamine(s) and the dicarboxylic acid(s) can beprepared. Forming of a granulate salt material is generally no issuesince dry or substantially dry nylon salts are typically solid materialsat room temperature.

The salt suitably is a salt of a single diamine and a singledicarboxylic acid. The salt may also be a mixed salt, made from multiplecomponents. The mixed salt is suitably made of components comprising atleast two diamines and one dicarboxylic acid, or one diamine and atleast two dicarboxylic acids, or even at least two diamines and at leasttwo dicarboxylic acids.

Suitable, the diamine used for the salt in the process according to theinvention is selected from aliphatic diamines, or aromatic diamines, orany combination thereof. Examples of suitable aliphatic diamines areethylene diamine, 1,3-propane-diamine, 1,4-diaminobutane,1,5-pentanediamine, 2-methyl-1,5-pentanediamine, 1,6-hexanediamine,2,2,4-trimethyl-1,6-hexanediamine, 2,4,4-trimethyl-1,6-hexanediamine,cis-1,4-cyclohexanediamine, trans-1,4-cyclohexanediamine, isophoronediamine, 1,7-heptanediamine, 1,8-octanediamine, 1,9-nonanediamine,1,10-decanediamine, 1,11-undecanediamine, 1,12-dodecanediamine,1,13-tridecanediamine, 1,14-tetradecanediamine, 1,15-pentadecanediamine,1,16-hexadecanediamine, 1,17-heptadecanediamine, 1,18-octadecanediamine,1,19-nonadecanediamine, and 1,20-eicosanediamine. Examples of suitablearomatic diamines are 1,4-diaminobenzene, 1,3-diaminobenzene,1,4-naphthalenediamine, 1,5-naphthalenediamine 2,6-naphthalenediamine,meta-xylenediamine and para-xylenediamine.

Preferably, the diamine comprises at least an aliphatic C2-C10 diamine,i.e. an aliphatic diamine with from 2 up to and including 10 carbonatoms. The advantage thereof is that the salt resulting from the diaminehas a higher solubility in water. More preferably, the diamine comprisesone or more than one aliphatic C2-C10 diamine in an amount of at least50 mole %, relative to the total molar amount of diamine. Still morepreferably, the amount of aliphatic C2-C10 diamine is at least 75 mole%, relative to the total molar amount of diamine.

In a particular embodiment, the diamine comprises one or more than onealiphatic C4-C6 diamine in an amount of at least 50 mole %, relative tothe total molar amount of diamine. More particular, the amount ofaliphatic C4-C6 diamine is at least 75 mole %, relative to the totalmolar amount of diamine. The aliphatic C4-C6 diamine suitably consistsof a diamine selected from 1,4-diaminobutane, 1,5-pentanediamine,2-methyl-1,5-pentanediamine and 1,6-hexanediamine, or any mixturethereof; preferably selected from 1,4-diaminobutane, 1,5-pentanediamineand 1,6-hexanediamine, or any mixture thereof.

Suitably, the dicarboxylic acid is selected from aromatic dicarboxylicacid, or aliphatic dicarboxylic acid, or any combination thereof.

Suitable examples of aromatic dicarboxylic acids are terephthalic acid,isophthalic acid, 2,6-naphthalene dicarboxylic acid, 4,4′-diphenyldicarboxylic acid.

For the aliphatic dicarboxylic acids suitable examples are oxalic acid,succinic acid, 1,5-pentanedicarboxylic acid, adipic acid,1,7-heptanedioic acid, 1,8-octanedioic acid, azaleic acid, sebacic acid,1,11-undecanedioic acid, 1,12-dodecanedioic acid, 1,13-tridecanedioicacid, 1,14-tetradecanedioic acid, 1,15-pentadecanedioic acid,1,16-hexadecanedioic acid, 1,17-heptadecanedioic acid,1,18-octadecanedioic acid, 1,19-nonadecanedioic acid, and1,20-eicosanedioic acid.

Preferably, the dicarboxylic acid comprises a C4-C10 aliphaticdicarboxylic acid, i.e. an aliphatic dicarboxylic acid with from to 4 upto and including 10 carbon atoms. The advantage thereof is that the saltresulting from the dicarboxylic acid has a higher solubility in water,allowing the use of a higher salt concentration and less water to beevaporated. More preferably, the dicarboxylic acid comprises adipicacid.

Also preferably, the dicarboxylic acid comprises an aromaticdicarboxylic acid. The advantage of this selection is that thetemperature conditions applied in the fluidized bed spray granulationcan be varied over a wider range. For example, due to the higher meltingpoint of salts comprising an aromatic dicarboxylic acid the temperatureof the heated air applied for drying may be higher. Examples of saltsthat can be prepared with the process according to the invention includethe following semi-aromatic salts: XT, XI, XT/XI, XT/X6, XT/Y6 andXT/YT, and any combinations thereof, wherein T represents terephthalicacid, I represents isophthalic acid, 6 represents adipic acid, and X andY represent diamines.

In a preferred embodiment X and Y are different diamines independentlyfrom each other chosen from aliphatic diamines from the group consistingof ethylene diamine, 1,4-diaminobutane, 1,5-pentanediamine,2-methyl-1,5-pentanediamine, 1,6-hexanediamine,2,2,4-trimethyl-1,6-hexanediamine, 2,4,4-trimethyl-1,6-hexanediamine,cis-1,4-cyclohexanediamine, trans-1,4-cyclohexanediamine,1,8-octanediamine, 1,9-nonanediamine, and 1,10-decanediamine.

Other examples of salts that can be prepared with the process accordingto the invention include the following aliphatic salts: 46, 66, 46/66,66/68, and 410, and any combinations thereof.

The salts may also be mixed salts, for example comprising a mixture ofcomponents from any of the mentioned semi-aromatic salts XT, XI, XT/XI,XT/X6, XT/Y6 and XT/YT and any of the aliphatic salts 46, 66, 46/66,66/68, and 410.

In a particular embodiment of the process according to the invention thediamine comprises an aromatic diamine and/or an aliphatic C2-C10 diamineand the dicarboxylic acid comprises a C4-C10 aliphatic dicarboxylic acidand/or an aromatic dicarboxylic acid.

In a preferred embodiment of the invention, the dicarboxylic acid in thesalt comprises terephthalic acid, for example in an amount of at least50 mole %, more particular at least 75 mole %, or even consists ofterephthalic acid. The mole % is herein calculated as mole %, relativeto the total molar amount of dicarboxylic acid. The advantage is that incase of occurrence of residual terephthalic remaining in the polyamideproduced upon direct solid state polymerization, such residual acid canbe reduced more effectively by post dosing of diamine, than for similarsalts prepared via a dry route.

In a more preferred embodiment, the dicarboxylic acid consists for atleast 75 mole % of terephthalic acid, and the diamine consists for atleast 75 mole % of 1,6-hexamethylediamine (HMDA) and/or1,4-diaminobutane (DAB) and/or 1,5-pentanediamine.

Preferably the diamine comprises 1,6-hexamethylediamine (HMDA) and1,4-diaminobutane (DAB) in a molar ratio DAB/HMDA in the range of25:7-45:55, or even better 30:70-40:60, as this gives rise to apolyamide with eutectic melting point.

Also preferable, the diamine comprises 1,6-hexamethylediamine (HMDA) and1,5-pentanediamine (PD) in a molar ratio PD/HMDA in the range of30:70-70:30, or even better 40:60 40:60-60:40.

Even more preferred the dicarboxylic acid consists for 95-100 mole % ofterephthalic acid, and the diamine consists for at least 95-100 mole %of 1,6-hexamethylediamine (HMDA), 1,4-diaminobutane (DAB) or1,5-pentanediamine. Suitably, the corresponding polyamides are eitherhomopolymers PA-4T, PA-5T or PA-6T, or copolymers thereof such asPA4T/5T, PA-4T/6T and PA-5T/6T, or PA-4T/5T/6T.

In the process according to the invention, the dicarboxylic acid and thediamine do not need to be present in exact equimolar amounts. Excess ofdicarboxylic acid may be compensated by dosing extra diamine duringpolymerization. Excess of diamine is generally already lost duringfluidized bed spray granulation and otherwise may result in diamine lossduring polymerization but such salts can still be polymerized to highmolecular weight polyamides. Preferably, the dicarboxylic acid and thediamine are present in a molar ratio in the range of 0.95:1-1.05:1,preferably 0.98:1-1.02:1, more preferably 0.99:1-1.01:1.

The invention also relates to a nylon salt. The nylon salt according tothe invention is a granulate salt material comprising a diammoniumdicarboxylate (salt obtainable by fluidized bed spray granulation. Thenylon salt according to the invention has the advantages in a DSSPprocess, as mentioned above.

The granulate material formed in said fluidized bed spray granulationprocess consists of agglomerated particles. These particles tend to havea blackberry like structure, in particular in the earlier stage of theagglomeration process when the size of the particles is still relativesmall, or have a more rounded off, when growing larger, likewise withmore spherical like structure (see FIGS. 1, 1 (b) and 2).

In contrast to the granulate salt material obtained by the processaccording to the invention, salt in particulate form obtained by spraydrying typically has a kind of puffed open popcorn type of structure orlittle droplets that are burst open (see FIG. 3 ). Salt in particulateform obtained by a dry solid process comprising dosing diamine to soliddicarboxylic acid shows a potato shape with a cracked surface like driedmud with many sub-micron crystallites (see FIG. 4 ). Salts precipitatedfrom aqueous solution generally show a (partially) crystalline, needleshape, or very fine powder, or when obtained by evaporation may show biglumps.

Suitably, the granulate salt material according to the invention has aparticle size distribution with a median particle size (d50) in therange of 50-3000 μm, preferably in the range of 200-1000 μm, morepreferably 400-750 μm. Herein the particle size distribution is measuredwith laser granulometry by the method according to ISO 13320-1 at 20° C.

The granulate salt material according to the invention has furtherinteresting properties such as high density and low compressibility. Thecompressibility is determined by comparing the aerated bulk density(ABD) and the tapped bulk density (TBD). Each of the aerated bulkdensity (ABD) and the tapped bulk density (TPB) are measured by themethod according to ASTM D6393. Suitably, the compressibility, expressedby the ratio of (TBD-ABD)/TBD*100%, is at most 25%.

Suitably, the granulate salt material has a tapped bulk density in therange of 350-750 kg/m³; preferably the tapped bulk density is in therange of 450-700 kg/m³. The advantage of a higher tapped bulk density isthat a higher reactor loading can be applied in the DSSP process,resulting in a better capacity utilization.

The invention also relates to a process for preparing a nylon polymer.The process comprises direct solid state polymerization (DSSP) of adiammonium dicarboxylate salt (i.e. a nylon salt), wherein the salt is agranulate salt material prepared by fluidized bed spray granulation.

In such process, the salt is polymerized by applying a suitable heatingprofile, in which the temperature remains below the melting temperatureof the salt. Preferably, during the direct solid state polymerization,the granular salt material is heated to elevated temperature, andwherein said temperature is kept below the melting temperature of thesalt minus 10° C., preferably below the melting temperature of the saltminus 15° C.

According to one embodiment of the invention, the granulate diammoniumdicarboxylate salt material obtained by fluidized bed spray granulationis used as such in the direct solid state polymerization. Herein ismeant that the salt as obtained from the fluidized bed spraygranulation, has not been subjected to a pelletizing step or any othershaping step, nor has been dissolved or dispersed in a liquid medium,before or during the direct solid state polymerization.

Suitably, during the direct solid state polymerization the granulatesalt material and the resulting nylon polymer are mechanically agitated.Mechanical agitation may be accomplished, for example, by rotation or bystirring with an agitator, or a combination thereof. In a preferredembodiment, said DSSP process is carried out in a reactor comprising anagitator or in a rotatable reactor, e.g. tumble drier. The process hasthe advantages of limited attrition and limited scaling as describedabove.

The polyamide prepared by the DSSP process suitably is a copolyamide ofone or more semi-aromatic polyamides and one or more aliphaticpolyamides; or one or more aromatic polyamides and one or more aliphaticpolyamides; or one or more semi-aromatic polyamides and one or morearomatic polyamides; and combinations thereof. Herein the constitutingpolyamide components are suitably selected from various aromaticpolyamides, semi-aromatic polyamides and aliphatic polyamides andprepared from the corresponding salts comprising the corresponding mixeddiamines and/dicarboxylic acids, as described herein above.

The invention also relates to a polyamide granulate material. The saidgranulate material comprises a polyamide polymer and is obtained bydirect solid state polymerization of a diammonium dicarboxylate saltprepared by fluidized bed spray granulation process. The advantagesthereof are that the said polyamide material is not only prepared in anefficient manner by DSSP, it also requires less extensive steps forcleaning and de-dusting and is more easily discharged than correspondingpolyamides with the same composition obtained from salts prepared byother routes. Moreover, the polyamide granulate material is good flowingand easy to handle in further processing

Herein the polyamide granulate material can be any polyamide granulatematerial obtained from any diammonium dicarboxylate salt as describedabove and obtained by the process according to the invention or anypreferred or particular embodiment thereof as described above.

The polyamide granulate material according to the invention comprises anAABB polyamide, comprising alternating AA and BB repeating units,wherein AA represents the repeating units derived from diamine and BBrepresents the repeating units derived from dicarboxylic acid.

Next to the repeating units derived from diamine and repeating unitsderived from dicarboxylic acid, the polyamide may comprise some mineramounts of other repeating units, i.e. repeating units derived fromother monomers. For example small amounts of monofunctional monomersand/or higher functional monomers may be included in the preparation ofthe salt and the polymerization thereof. Examples of such monomers aremonofunctional amines, monofunctional carboxylic acids, triamines andtrifunctional carboxylic acids. Suitably such other monomers aredissolved, next to the diamine and the dicarboxylic acid, in the aqueousmedium used for the fluidized bed spray granulation. Preferably, theamount of such other monomers is low, for example below 5 mole %, morepreferably below 2 mole %, and the polymer consists essentially ofrepeat units derived from diamines and dicarboxylic acids, i.e. for95-100 mole %, preferably for 98-100 mole %. Herein the mole % isrelative to the total molar amount of monomeric repeating units in thepolyamide.

Examples of polyamides that can be prepared with the process accordingto the invention include the following semi-aromatic polyamides: PA-XT,PA-XI, PA-XT/XI, PA-XT/X6, XT/Y6 and PA-XT/YT, and any copolymerthereof, wherein T represents terephthalic acid, I representsisophthalic acid, 6 represents adipic acid, and X and Y representdiamines.

In a preferred embodiment, X and Y represent different diaminesindependently from each other chosen from aliphatic diamines from thegroup consisting of ethylene diamine, 1,4-diaminobutane, 1,5-diamine,2-methyl-1,5-pentanediamine, 1,6-hexanediamine,2,2,4-trimethyl-1,6-hexanediamine, 2,4,4-trimethyl-1,6-hexanediamine,cis-1,4-cyclohexanediamine, trans-1,4-cyclohexanediamine,1,8-octanediamine, 1,9-nonanediamine, and 1,10-decanediamine, and anycopolymer thereof.

Eventually the salt, and the polyamide prepared from the salt, maycomprise three or more diamines. Suitably these diamines are selected inpart, or even all, from the above group.

In a more preferred embodiment, the polyamides are derived from saltsbased on dicarboxylic acid consisting for at least 75 mole % ofterephthalic acid, and diamine consisting for at least 75 mole % of1,6-hexamethylediamine (HMDA) and/or 1,4-diaminobutane (DAB) and/or1,5-pentanediamine. Preferably the diamine comprises1,6-hexamethylediamine (HMDA) and 1,4-diaminobutane (DAB) in a molarratio DAB/HMDA in the range of 25:75-45:55, or even better 30:70-40:60.Also preferable, the diamine comprises 1,6-hexamethylediamine (HMDA) and1,5-pentanediamine (PD) in a molar ratio PD/HMDA in the range of30:70-70:30, or even better 40:60-60:40.

Even more preferred the dicarboxylic acid consists for 95-100 mole % ofterephthalic acid, and the diamine consists for at least 95-100 mole %of 1,6-hexamethylediamine (HMDA), 1,4-diaminobutane (DAB) or1,5-pentanediamine. Suitably, the corresponding polyamides are eitherhomopolymers PA-4T, PA 5T or PA6T, or copolymers thereof such asPA-4T/5T, PA-4T/6T and PA5T/6T, and PA4T/5T/6T.

Other examples of polyamides that can be prepared with the processaccording to the invention include the following aliphatic polyamides:PA-46, PA-66, PA-46/66, PA-66/68, and PA-410, and any copolymer thereof.

The polyamide may also be a copolyamide such as, for example, acopolyamide of one or more of the semi-aromatic polyamides and one ormore of the aliphatic polyamides; one or more of the aromatic polyamidesand one or more of the aliphatic polyamides; or one or more of thesemi-aromatic polyamides and one or more of the aromatic polyamides; andcombinations thereof.

Preferably the copolyamide is a copolyamide of one or more of thementioned semi-aromatic polyamides and one or more of the said aliphaticpolyamides. Examples thereof are PA-XT/YT/X6/Y6, wherein T representsterephthalic acid, 6 represents adipic acid, and X and Y representdifferent diamines. In these copolymers the diamine may also suitablycomprise three or more diamines.

The granulate salt material according to the invention may also be usedin a process wherein a diammonium dicarboxylate salt is polymerized in aliquid state. Herein the diammonium dicarboxylate salt suitablycomprises a granulate salt material prepared by fluidized bed spraygranulation and the granulate salt material is either molten ordissolved. For example, a 66 salt prepared by fluidized bed spraygranulation can be advantageously used in a conventional meltpolymerization process to produce polyamide-66, or a copolymer thereof.Another example is the polymerization of such a salt in a solutionprocess in an aqueous solution to prepare a prepolymer, followed byisolation of the prepolymer and subsequent solid state post condensationto convert the prepolymer in a polyamide polymer.

EXAMPLES

The invention is further illustrated with the following examples andcomparative experiments.

Materials

For the raw materials, terephthalic acid, adipic acid,1,4-butanediamine, 1,5-pentanediamine and hexamethylenediamine,industrial grades were used.

Methods Aerated Bulk Density (ABD) and Tapped Bulk Density (TBD)

The ABD and TBD were measured by the method according to ASTM D6393-08(“Standard Test Method for Bulk Solids Characterization by CarrIndices”, ASTM International, West Conshocken, PA, DOI:10.1520/D6393-08) with a Hosokawa Powder Tester at 20° C.

Residual Water Content

The residual water content of a salt was determined by drying of thesalt during 24 hours at 105° C. and under a vacuum of 100 mmHg, i.e.equal to an absolute pressure of 0.01 bar. The weight of the salt wasmeasured before and after drying. The residual water content wascalculated from the weight loss expressed as weight percentage relativeto the weight of the granulate salt material before drying.

Compositions of Salts and Polymers

The composition of the salt with regard to ratio of diamines wasdetermined by ¹H-NMR determined by titration of a salt solution inwater. The ratio of dicarboxylic acid/diamine was calculated form theexcess of diamine determined by titration of a salt solution in water.The number of end groups, i.e. residual amine functional groups andresidual carboxylic acid functional groups was determined by ¹H-NMR.

Particle Size Distribution by Laser Diffraction

The particle size distribution of granulate material, and d10, d50 andd90 values, were measured by laser granulometry according to ISO 13320-1with a Sympatec Helos (H0024) & Rodos apparatus at 20° C. with anapplied pressure of 0.5 bar and an measured under-pressure in theventuri of 25 mbar. From the results the Span, defined by the ratio of(d90-d10)/d50, is calculated.

Flow Behavior in Shear Test

The flowability was measured by the method according to ASTM StandardD6773-08 (“Standard Shear Test Method for Bulk Solids Using the SchulzeRing Shear Tester”, ASTM International, West Conshocken, PA, DOI:10.1520/D6773-08). The shear test was performed with a Schulze RingshearTester at 20° C. with a consolidation stress of 3 kPa. The measurementwas started immediately after filling of the tester. The flow behaviorwas performed on freshly prepared material, stored in a dry environment.

Experiments Preparation of Salt Solution

For the preparation of salts by spray drying and spray granulationexperiments, salts solutions were used, which were prepared as follows.A solution of terephthalic acid, 1,4-butanediamine and/orhexamethylenediamine in water were prepared by adding a1,4-butanediamine solution in water, a hexamethylenediamine solution inwater and terephthalic acid to a complementary amount of water understirring and heating. The liquid mixture was heated to 60° C., uponwhich a clear solution was obtained. Upon cooling during storage, thedissolved salt partially crystallized out, resulting in a slurry. Uponreheating the partially crystallized salt dissolved again.

Solutions were prepared with concentrations between 28 and 50 wt. %salt. The concentration is calculated as the wt. % of all saltcomponents, relative to the total weight of the solution. The appliedratio between 1,4-butanediamine and hexamethylenediamine was in therange of 36/64 and 42/58. The molar ratio between terephthalic acid anddiamine was preset at 0.995, aiming at a slight excess of diamine.

Dry Salt Preparation

Dry salt preparation was carried out in a reactor equipped with amechanical stirrer. Solid terephthalic acid (245 kg) was charged intothe reactor, stirred and heated to 60° C. A mixture of 48 kg1,4-butanediamine and 110 kg hexamethylenediamine was prepared in adosing vessel and heated to 60° C. The diamine mixture was dosed to theterephthalic acid under stirring, while maintaining the temperature inthe reactor between 30° C. and 95° C. After completion of the dosing thereactor content was kept at 95° C. while remaining stirring. The totalbatch cycle time of the salt formation step was 32 hours. Then thereactor content was cooled and discharged.

Two batches of terephthalic acid were used, one was an industrial gradeused as received and the other one was a batch with smaller particlesize obtained by sieving.

Properties of the salts prepared by this route, referred to asDry-salt-1 and Dry salt-2 are shown in Table 1.

Spray Drying

Spray-drying was carried out in a spray dryer with an annular channelwith a nitrogen loop and equipped with high pressure nozzles and acyclone for separation of the fines. A 28 wt. % salt solution was heatedto 50° C. and the gas inlet temperature was set at 160° C. The nozzlewas pressurized to 55 bar. The product was obtained as a free flowingpowder. Properties of the spray dried salt, referred to as SD salt, areshown in Table 1.

Fluidized Bed Spray Granulation SG-1 and SG-2

Fluidized bed spray granulation was performed on pilot scale in a GlattAGT 400 apparatus, comprising a spray column provided with spray nozzlesat the top and center of the spray column and a classification channelat the center in the bottom. A salt solution was either freshly preparedor taken from storage and heated to 50° C.-60° C. The heating wasperformed by providing an electrical heating jacket around a storagebin. A fluidized bed of salt powder was prepared using a small amount ofsalt powder prepared on lab scale in a Glatt GPCC 3.1 apparatus. In thelab scale experiment, an initial bed was prepared from salt particlesobtained by spray drying. The gas inlet temperature in the experiment onpilot scale was varied between 100° C. and 150° C. The flow offluidization/heating gas was varied, thereby varying the residence timeof the particles and allowing the particles to grow to larger particlesize (SG-1) or smaller particle sized (SG-2). The bed temperature wascontrolled to be between 40° C. and 55° C. Properties of two examples ofgranulate salt materials (SG1 and SG2, with a monomer compositionsimilar to in the comparative examples) obtained by the above fluidizedbed spray granulation process are shown in Table 1.

SG-3

The procedure for SG-1 was repeated with a salt solution with a monomercomposition based on terephthalic acid, 1,5-pentanediamine and1,6-hexanediamine. The salt obtained had a residual water content of 5.2wt. % and was dried afterwards to a residual water content of 2 wt. %.The properties of the granulate salt material SG-3 thus obtained areshown in Table 1.

SG-4

The procedure for SG-2 was repeated with a salt solution with a monomercomposition based on terephthalic acid, adipic acid, 1,4-butanediamineand 1,6-hexanediamine. The salt obtained had a residual water content of6.4 wt. % and was dried afterwards to a residual water content of 1.5wt. %. The properties of the granulate salt material SG-4 thus obtainedare shown in Table 1.

TABLE 1 Analytical results of granulate salt materials obtained by spraydrying, by dry salt preparation and by fluidized bed spray granulationSD-salt Dry salt-1 Dry salt-2 SG1 SG2 SG3 SG4 DAB/HMDA ratio 42/58 ^(a))36/64 ^(a)) 36/64 ^(a)) 39/61 ^(a)) 39/61 ^(a)) 29/71 ^(b)) 31/69 ^(a))Acid/Diamine ratio 0.995 0.996 0.995 0.997 0.996 0.998 0.990 D10 (μm) 257 5 352 134 222 83 D50 (μm) 89 190 79 560 301 359 243 D90 (μm) 198 900377 739 467 527 372 Span 1.94 4.70 4.74 0.69 1.10 0.85 1.19 Aerated Bulkdensity (kg/m³) 223 367 679 535 Tapped Bulk density (kg/m³) 629 731 593Compressibility (%) 42 7 10 Residual water content (wt. %) 1.6 1.8 1.83.7 0.7 5.2 (then 6.4 (then dried to 2.0) dried to 1.5) Flow behaviorFree Free Free Free Free Free Free flowing flowing flowing flowingflowing flowing flowing ^(a)) DAB/HMDA ratio; ^(b)) PD/HMDA ratio.

Comparative Experiment A

A direct solid state polymerization (DSSP) was carried out in a 1 literlab scale reactor equipped with a mechanical stirrer. During heating ofthe salt in the DSSP process, polymerization takes place, the salt istransformed and a polymer is formed. During the polymerization somediamine can be lost, resulting in some excess of dicarboxylic acid.

The reactor was charged with 300 gram of SD salt, i.e. the salt preparedby spray drying. The salt was stirred at a rotation speed of the stirrerof 30 rpm and inertized with nitrogen gas flow of 3 liter/hours at roomtemperature for 3 hours and then heated while kept under stirred motion,applying a temperature profile, wherein the salt was heated with aheating rate of 1° C./m in to 250° C., kept at that temperature for 16hours. Then, the reactor content was cooled and discharged from thereactor. The following was observed: a lot of fines was formed; the flowbehavior was extremely bad, prohibitive for a large scale experiment.The discharged material was collected and analyzed, the internal of thereactor showed hardly any fouling or scaling.

Other Experiments on Lab Scale

Comparative Experiment A was repeated with the salt materials Dry-salt-1and Dry salt-2, and SG-1 and SG-2, which showed all better results thanComparative Experiment A in that less fines were produced and flow ofthe materials was much better. In case of the salts obtained by thedry-route slightly more fouling was observed. Since these runs were OK,experiments on larger scale could be performed.

Comparative Experiment B: Direct Solid State Polymerization ofDry-Salt-1

Direct solid state polymerization (DSSP) at larger scale was carried outin a 1 m3 reactor equipped with a mechanical stirrer. Prior to theexperiment the reactor was cleaned and inertized with nitrogen gas. Thenthe reactor was charged with 325 kg of salt powder Dry-salt-1 preparedby dry route as described before. The salt was stirred at a rotationspeed of the stirrer of 16.4 rpm and heated while kept under stirredmotion, applying a temperature profile, wherein the salt was heated witha heating rate of 0.5° C./min to 180° C., kept at that temperature for 4hours, then further heated with a heating rate of 10° C./min to 265° C.and kept at that temperature for 14 hours. During the polymerizationsome diamine might have been lost, resulting in some excess ofdicarboxylic acid. Additional diamine was dosed to the reactor contentat the end of the reported temperature profile and maintained at thattemperature until a VN of 70-74 was obtained. The reactor content wascooled and discharged from the reactor. The discharged material wascollected and analyzed, and the internal of the reactor was inspectedfor fouling and any other defects. 220 kg of polymer came out of thereactor (68 wt. % yield, max. theoretical yield is 87 wt. %), the restof the material stayed behind as scaling and fouling of the reactor walland internals.

Comparative Experiment C—Direct Solid State Polymerization of Dry-Salt-1

Comparative Experiment B was repeated except that Dry-salt-2 was usedinstead of Dry-salt-1. The yield was 65 wt. %, (max. theoretical yieldis 87 wt. %), the rest of the material stayed behind as scaling andfouling of the reactor wall and internals.

Example I: Direct Solid State Polymerization of SG-1

Comparative Experiment B was repeated except that SG-1 was used insteadof Dry-salt-1. The yield was 86 wt. %. The internal of the reactorshowed hardly any fouling or scaling.

Example II: Direct Solid State Polymerization of SG-2

Comparative Experiment B was repeated except that SG-2 was used insteadof Dry-salt-1. Also the amount of salt charged to the reactor wasincreased to 450 kg. The yield was 86 wt. %. the internal of the reactorshowed hardly any fouling or scaling.

Properties and analytical results of the granulate polymer materialobtained by DSSP for the Examples (EX 1-2) and Comparative Experiments(CE A-C) are shown in Table 2.

TABLE 2 Analytical results of polyamide products obtained by DSSP CE-ACE-B CE-C EX-I EX-II Salt SD Dry Dry SG1 SG2 salt salt-1 salt-2 Acid endgroups 455 153 36 588 129 (meq/kg) Amine end groups <10 102 120 <10 25(meq/kg) Free acid (wt. %) 1.04 0.50 0.45 0.48 0.03 VN n.m. 85 69 71 75Tm n.m. 338 343 344 344 D10 (μm) n.m. 20 8 314 136 D50 (μm) n.m. 90 80490 298 D90 (μm) n.m. 330 173 693 464 Span n.m. 3.44 2.06 0.77 1.10 Bulkdensity n.m. 435 589 476 (kg/m³) Aerated Bulk density n.m. 612 663 606(kg/m³) Tapped Compressibility (%) n.m. 29 11 21 Flow behavior Very PoorPoor Free Free bad flowing flowing Mass balance (%) n.m. 68 65 86 86

The results show that with the process according to the invention lessfouling and agglomeration occurs. Moreover, the particle sizedistribution and the average particle size remains about the same, andalso the Span factor remains about unaltered, indicative that a limitedamount of fines is formed. Also the polymer material obtained with theprocess has retained free flowing properties.

The results for CE-B show a particle size distribution with a muchsmaller median particle size, and less large particles. Also a lot ofscaling in the reactor was observed, which consisted of caked fineparticles. Therefore, these fines are not measured in the remainingpowder. These observations are indicative for the occurrence ofextensive attrition, resulting of pulverization of the large particlesin the salt into fines.

The results further show an excess of diamine end groups in the polymerof CE-C, in combination with a substantial amount of residualterephthalic acid. This illustrates that the post dosing was effectivein compensating any loss in diamine, but not in reducing the amount ofresidual terephthalic acid. The polymer of EX-I shows a large excessdicarboxylic acid end groups, which is an indication that thepost-dosing of diamine was incomplete. Nevertheless, the amount ofresidual terephthalic acid is not higher than in CE-B. The polymer ofEX-II also shows an excess dicarboxylic acid end groups, but muchsmaller than for EX-I, which is an indication that the post-dosing ofdiamine was more complete for EX-II than for EX-I. Meanwhile, the amountof residual terephthalic acid for EX-II is much lower than for both EX-Iand CE-B.

Example III: Direct Solid State Polymerization of SG-3

Comparative Experiment A was repeated except that SG-3 was used insteadof the SD salt, and the amount of salt charged to the reactor wasincreased to 400 g. Furthermore, a modified temperature profile wasapplied: the temperature was raised to 220° C. with a heating rate of 1C/min. Then the temperature was further increased to 240° C. with aheating rate of 2° C./hour, kept at 240° C. during 2 hours, heated to250° C. in 1 hour and kept at that temperature during 4 hours. Theproduct was obtained as a free flowing powder, the yield was 87 wt. %(compared to 91 wt. % theoretically) and the internal of the reactorshowed hardly any fouling or scaling. The free acid content was 0.13 wt.%, acid end groups amounted to 221 meq/kg and amine end groups below 10meq/kg. The bulk density aerated was 546 kg/m³ and the bulk densitytapped was 630 kg/m³.

Example IV: Direct Solid State Polymerization of SG-4

Comparative Experiment A was repeated except that SG-4 was used insteadof Dry-salt-1, the amount of salt charged to the reactor was increasedto 400 kg, and the temperature profile was modified as follows: thetemperature was raised to 210° C. with a heating rate of 1 C/min, andkept at that temperature for 15 hours. Then the temperature was furtherincreased to 230° C. in 1 hour, and kept at 230° C. during 8 hours,heated to 240° C. in 1 hour and kept at that temperature for 14 hours.The product was obtained as a free flowing powder, the yield was 85 wt.% (compared to 86.8 wt. % theoretically). The internal of the reactorshowed hardly any fouling or scaling. The content of free acid was 0.11wt. %, acid end groups amounted to 245 meq/kg and amine end groups wereabout 19 meq/kg. The bulk density aerated was 489 kg/m³ and the bulkdensity tapped was 590 kg/m³.

These results of Examples III and IV show that the salts according tothe invention a higher filler load can be applied compared to the SDmaterial of Comparative Experiment A, while still obtaining a freeflowing powder in a high yield, with a very low residual free acidcontent.

The invention claimed is:
 1. A process for preparing a polyamidecomprising conducting direct solid state polymerization (DSSP) of adiammonium dicarboxylate salt, wherein the diammonium dicarboxylate saltis a granulate salt material prepared by fluidized bed spray granulationof an aqueous solution or slurry comprising diammonium dicarboxylatesalt dissolved or dispersed in an aqueous medium, and wherein thediammonium dicarboxylate salt has the following physical properties: (a)a particle size distribution with a median particle size (d50) in therange of 200-1000 μm, measured by laser granulometry with the methodaccording to ISO 13320-1 at 20° C., and (b) a tapped bulk density in therange of 350-750 kg/m³, measured by the method according to ASTM D6393.2. The process according to claim 1, wherein the granulate salt materialis a nylon salt.
 3. The process according to claim 2, wherein the directsolid state polymerization (DSSP) is carried out in a reactor comprisingan agitator, or in a rotatable reactor, or a combination thereof, andwherein during the polymerization the granulate salt material and theresulting nylon polymer are mechanically agitated.
 4. The processaccording to claim 1, which further comprising dosing additional diamineat least during some part of the direct solid state polymerization orafter the direct solid state polymerization.
 5. The process according toclaim 1, wherein the polyamide is: (i) an aromatic polyamide selectedfrom PA-MXDT and PA-PXDT; (ii) a semi-aromatic polyamide selected fromthe group consisting of PA-XT, PA-XI, PA-XT/XI, PA-XT/X6, XT/Y6 andPA-XT/YT, and any copolymer thereof, wherein T represents terephthalicacid, I represents isophthalic acid, 6 represents adipic acid, and X andY represent diamines, independently from each other chosen fromaliphatic diamines from the group consisting of ethylene diamine,1,3-propanediamine, 1,4-diaminobutane, 1,5-pentanediamine,2-methyl-1,5-pentanediamine, 1,6-hexanediamine,2,2,4-trimethyl-1,6-hexanediamine, 2,4,4-trimethyl-1,6-hexanediamine,cis-1,4-cyclohexanediamine, trans-1,4-cyclohexanediamine,1,8-octanediamine, 1,9-nonanediamine, and 1,10-decanediamine; (iii) analiphatic polyamide selected from the group consisting of PA-46, PA-66,PA-46/66, PA-66/68, and PA-410, and any copolymer thereof; or (iv) acopolyamide of one or more of the semi-aromatic polyamides and/or one ormore of the aliphatic polyamides, and/or one or more of the aromaticpolyamides.
 6. A polyamide granulate material which is obtained bydirect solid state polymerization (DSSP) of a diammonium dicarboxylatesalt, wherein the diammonium dicarboxylate salt is a fluidized bed spraygranulation product of an aqueous solution or an aqueous slurrycomprising a diammonium dicarboxylate salt dissolved or dispersed in anaqueous medium, and wherein the diammonium dicarboxylate salt has thefollowing physical properties: (a) a particle size distribution with amedian particle size (d50) in the range of 200-1000 μm, measured bylaser granulometry with the method according to ISO 13320-1 at 20° C.,and (b) a tapped bulk density in the range of 350-750 kg/m³, measured bythe method according to ASTM D6393.
 7. The polyamide granulate materialaccording to claim 6, wherein the polyamide granulate material comprisesa polyamide polymer consisting of: (i) an aromatic polyamide selectedfrom PA-MXDT and PA-PXDT; (ii) a semi-aromatic polyamide selected fromthe group consisting of PA-XT, PA-XI, PA-XT/XI, PA-XT/X6, XT/Y6 andPA-XT/YT, and any copolymer thereof, wherein T represents terephthalicacid, I represents isophthalic acid, 6 represents adipic acid, and X andY represent diamines, independently from each other chosen fromaliphatic diamines from the group consisting of ethylene,1,3-propanediamine, diamine, 1,4-diaminobutane, 1,5-pentanediamine,2-methyl-1,5-pentanediamine, 1,6-hexanediamine,2,2,4-trimethyl-1,6-hexanediamine, 2,4,4-trimethyl-1,6-hexanediamine,cis-1,4-cyclohexanediamine, trans-1,4-cyclohexanediamine,1,8-octanediamine, 1,9-nonanediamine, and 1,10-decanediamine; (iii) analiphatic polyamide selected from the group consisting of PA-46, PA-66,PA-46/66, PA-66/68, and PA-410, and any copolymer thereof; or (iv) acopolyamide of one or more of the semi-aromatic polyamides and/or one ormore of the aliphatic polyamides, and/or one or more of the aromaticpolyamides.
 8. A process for preparing a polyamide comprisingpolymerizing a diammonium dicarboxylate salt in a liquid state, whereinthe diammonium dicarboxylate salt comprises a granulate salt materialprepared by fluidized bed spray granulation that is either molten ordissolved, and wherein the granulate salt material is the spraygranulation product of an aqueous solution or slurry comprisingdiammonium dicarboxylate salt dissolved or dispersed in an aqueousmedium, and wherein the granulate salt has the following physicalproperties: (a) a particle size distribution with a median particle size(d50) in the range of 200-1000 μm, measured by laser granulometry withthe method according to ISO 13320-1 at 20° C., and (b) a tapped bulkdensity in the range of 350-750 kg/m³, measured by the method accordingto ASTM D6393.